Offshore petroleum drilling and producing operations commonly are conducted from bottom-founded offshore structures which typically are constructed by welding large diameter steel tubular members together to form a trussed framework known as a "steel jacket structure". Such steel jacket structures may be of either the fixed type, which rigidly resist environmental forces resulting from wind, waves, and currents or the compliant type which yield to the environmental forces in a controlled manner; and either type may be as much as 1,000 feet (305 meters) or more in height.
Steel jacket structures are subject to damage from a variety of causes. For example, a steel jacket structure must be capable of withstanding not only the relatively infrequent impacts of very large waves caused by severe storms, but also the cumulative effects of repeated impacts of smaller waves which are present under most sea states. These smaller waves tend to cause vibration in the individual tubular members of the jacket structure. Vibration of a tubular member may result in the formation of fatigue cracks at or near the welded ends of the member which, if undetected and uncorrected, could ultimately result in a fatigue failure.
Another potential cause of damage to a steel jacket structure is corrosion. The corrosion that is of the greatest concern is that which occurs along the weld at one of the jacket structure's primary welded joints. This type of corrosion, known as "knife-edge corrosion", occurs at the juncture of the weld metal and the heat-affected-zone of the base metal. Knife-edge corrosion acts as a stress raiser thereby reducing the welded joint's ability to withstand fatigue loadings.
An offshore petroleum reservoir may have a producing life in excess of 30 years. The offshore structure used to produce such a reservoir must have a useful life which exceeds the anticipated producing life of the reservoir. Since the damage resulting from vibration and corrosion generally occurs over a substantial period of time, it is desirable that the welded joints of an offshore structure be inspected periodically so that damage may be detected and corrective action may be taken before a failure occurs.
In shallow waters, the welded joints of a steel jacket structure typically are inspected visually by a diver. In deeper waters, a manned submersible vessel may be used to facilitate visual inspection of the welded joints, or an unmanned submersible vessel may be equipped with television cameras to permit remote visual inspection of the joints.
As is well known in the art, certain nondestructive testing techniques may be used in addition to or in place of visual inspection to detect the presence of fatigue or corrosion cracks in the welded joints of a steel jacket structure. Such nondestructive testing techniques include, without limitation, electromagnetic, acoustic, magnetic particle, eddy-current, and ultrasonic testing.
In many areas of the world, such as offshore California and offshore Australia, a steel jacket structure is subject to a phenomenon known as "marine biofouling." As used herein and in the claims, "marine biofouling" means an organic encrustation comprising colonies or clusters of barnacles, mussels, hydroids, anemones, and other forms of marine life which forms under certain conditions on materials located in an offshore environment, such as the steel tubular members of a steel jacket structure. This organic encrustation grows in the form of a sheath or coating which may be as much as 24 inches or more thick.
Marine biofouling is harmful to an offshore structure in two different ways. First, the sheath of organic encrustation surrounding a tubular member of a steel jacket structure adds substantially to the weight and effective diameter of the member, thereby substantially increasing the wave and current forces which the steel jacket structure must resist. This is especially true since, as shown in FIG. 1 and more fully described below, marine biofouling tends to occur primarily near the surface of the body of water, which is the area where wave and current forces are generally the highest. Second, marine biofouling substantially obstructs efforts to periodically inspect the welded joints of an offshore structure, either visually or by any of the other available techniques, for the presence of fatigue or corrosion cracks. As noted above, the sheath of organic encrustation surrounding a tubular member may be as much as 24 inches or more thick. None of the presently known techniques for crack detection can be reliably used to inspect a heavily encrusted joint.
U. S. Pat. No. 4,415,293 issued Nov. 15, 1983 to Engel et al. discloses an effective solution to the first problem described above (i.e., marine biofouling resulting in a substantial increase in the wave and current forces which the steel jacket structure must resist). The Engel et al. patent discloses a method of preventing marine biofouling on the tubular members of a steel jacket structure by covering the tubular members with a sheath of marine growth-inhibiting material. The sheath comprises an outer layer of a non-ferrous material such as a copper-nickel alloy and an inner layer of an elastomeric polymeric material. The outer layer of non-ferrous material provides a source of a biocide or marine growth-inhibiting agent which substantially eliminates the attachment of marine biofouling encrustations.
The Engel et al., patent, however, does not offer an acceptable solution to the second problem discussed above (i.e., marine biofouling substantially obstructing inspection of the welded joints of the steel jacket structure). This is because application to a welded joint of a sheath of marine growth-inhibiting material in the manner taught by Engel et al. would itself obstruct inspection of the welded joint. Fatigue cracks in the underlying welded joint could not propagate through the ductile inner layer taught by Engel et al. and, therefore, could not be visually detected. Further, the ductile inner layer of Engel et al. would substantially negate the reliability of most commonly used underwater nondestructive testing methods for detecting cracks in the underlying welded joint.
Accordingly, a need exists for a method for substantially preventing marine biofouling of the welded joints in a steel jacket structure without obstructing the periodic inspection of such welded joints.